As the state-of-the-art is advanced in high-technology electronics in general, and computer systems in particular, more products based on this technology are becoming available to the consumer. For example, small-size and low-cost consumer electronics products can now be conveniently installed in vehicles, either as original equipment or in the aftermarket. A computerized vehicle navigation system which assists a driver to keep track of the location of the vehicle is one example of such a product. The vehicle navigation system may include a computer, a display and a data storage medium which stores information, such as a digitized map of streets. The computer will process the map data, together with other information representing the actual location of the vehicle, to provide a display of the map and a symbol showing the position of the vehicle relative to the streets being navigated. The data storage medium may be, for example, a tape cartridge or tape cassette that is read by tape read circuitry placed in the passenger compartment.
For computer systems which are used generally in many different environments, data are encoded or formatted on data storage media in a variety of ways, one of which, for example, is to add to the data, clock information that is decoded by the computer to properly process the data. Unique data encoding and decoding problems arise in the vehicle use environment, particularly if the computer system includes a tape cassette and a magnetic head to read the tape. One problem is that the vehicle use environment is a high shock and vibration environment, which can result in the tape undesirably moving relative to the head. This means that the data should be formatted to allow the read system to be able to read and acquire accurate reference information about the position of the tape, which is difficult in such an environment. Another problem of the vehicle use environment is in keeping the amount of tape read circuitry necessary to decode the data at a minimum and in keeping it simple, thereby minimizing cost to the consumer and facilitating repair or replacement, if necessary. Yet another problem is that a computer system used in the vehicle environment is subject to signal noise; therefore, the required bandwidth for the data format should be narrow since a narrow bandwidth reduces the noise power that can effect decoding of the data.